Spongiform neurodegeneration is a unique form of neurodegeneration characterized by vacuolation in neurons, neuronal cell death, and astrocytosis. Spongiform neurodegeneration is best known as the hallmark of prion disease, and this pathology is also present in the brains of patients suffering from Alzheimer's disease, diffuse Lewy body disease, and acquired immune deficiency syndrome (AIDS). The most common human prion disease is Creutzfeldt-Jakob disease (CJD), which occurs in sporadic, infectious, and inherited forms. The sporadic CJD form accounts for 85% of cases and is a late-onset (average age of onset = 60 years) neurodegenerative disorder of unknown cause. Although the prion protein has been extensively studied, the pathogenic mechanism underlying spongiform neurodegeneration remains elusive. Interestingly, a recent genetic study reveals that a null mutation in the gene encoding a novel protein called Mahogunin (Mgrn1) causes age-dependent, progressive spongiform neurodegeneration in mice that includes many features of prion disease but without accumulation of protease-resistant prion protein. At present, very little is known about the biological function of Mgrn1 and how loss of Mgrn1 function causes spongiform neurodegeneration. Mgrn1 contains a RING finger, a motif thought to be the key determinant of E3 ubiquitin-protein ligase activity. It has been speculated that loss of Mgrn1 function may cause spongiform neurodegeneration by impairing the ubiquitination and subsequent proteasomal degradation of yet-to-be-identified substrate(s) of Mgrn1. In contrast, the applicant's preliminary results have led to an intriguing hypothesis that Mgrn1 functions in a proteasome-independent, ubiquitin signalling pathway and suggest a novel mechanism by which defective ubiquitination may cause spongiform neurodegeneration. In this project, the applicant's group will follow up on these exciting results and use a combination of biochemical, cell biological, and molecular genetic approaches to investigate the cellular role of Mgrn1 E3 ligase, identify its substrates, and elucidate the molecular mechanism by which loss of Mgrn1 function causes age-dependent spongiform neurodegeneration. Completion of the proposed project should advance our understanding of the pathogenic mechanism underlying spongiform neurodegeneration and facilitate the development of new therapeutic strategies for treating age-related neurodegenerative disorders. PUBLIC HEALTH RELEVANCE: Spongiform neurodegeneration is most commonly associated with prion disease, but also occurs in patients suffering from age-related neurodegenerative disorders, such as Alzheimer's disease and diffuse Lewy body disease. The goal of the proposed research is to define the molecular pathogenic mechanism that causes age-dependent spongiform neurodegeneration. The results of the proposed studies will provide fundamental information needed for the development of effective therapeutics to combat age-related neurodegenerative diseases.